Image-representative signals (e.g. video signals) can be digitized, encoded, and subsequently decoded in a manner which substantially reduces the number of bits necessary to represent a decoded reconstructed image without undue or noticeable degradation in the reconstructed image. Coding methods that use transforms, for example discrete cosine transform ("DCT") or wavelet transforms, are well known and in widespread use. Video compression standards such as JPEG, MPEG-1, MPEG-2, H.261 and H.262 typically employ DCT-based techniques.
Techniques employing vector transform (VT) coding (see, for example, U.S. Pat. No. 5,436,985) can provide substantial improvements in coding efficiency over DCT-based methods used in the above referenced standards. In VT coding, an image (e.g. a video frame, a segmented video frame, or a motion compensated difference frame) is sub-sampled into multiple small images. Each small image is converted into a different format by using a transform such as the discrete cosine transform or a wavelet transform. The corresponding transform coefficients from the small images are grouped together to form vectors. Vector quantization is used to quantize and code those vectors.
Although techniques such as VT coding are advantageous, service providers can be faced with the problem of needing to retain hardware for encoding DCT-based and wavelet-based signals to serve those users who only have decoder equipment for such signals, while also investing in equipment that can encode the signals of more advanced techniques such as VT coding in order to serve those users who have the more advanced decoder equipment. The problem is analogous from a user standpoint, where a user having only DCT-based and/or wavelet based decoder hardware will be limited in capability of decoding signals encoded with more advanced techniques such as VT coding, whereas purchasers of VT decoding equipment will also want to be able to decode the signals encoded with DCT-based and wavelet-based encoders that will remain in use, but without having to purchase additional equipment for doing so.
It is among the objects of the present invention to provide improvements in encoding and decoding techniques and apparatus that are responsive to the problems just summarized. It is also among the objects of the invention to provide improved coding options and to provide an improved technique and apparatus for entropy coding.